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Wednesday, June 29, 2016

Perovskite Solar Cells - Some Recent Developments.

I have written the following article, which will be published later in the year in the journal Science Progress of which I am an editor. However, given the importance of its subject and the wide interest in perovskite solar cells, I am posting it here as a preview.

Introduction.

Perovskite solar
cellswere the subject of a current commentary, previously published in this journal1 These
are solar cells whose structure utilises a perovskite-type compound as the
light-harvesting active layer.1 The term 'perovskite solar cell'
originates from the ABX3 stoichiometry of the absorber materials,
which translates to a perovskite crystal structure (Figure 1). The most
extensively studied absorber material is methylammonium lead trihalide (CH3NH3PbX3;
where X is a Cl, Br or I atom), with a bandgap energy in the range of 1.5—2.3 eV.
The related material, formamidinum lead trihalide (H2NCHNH2PbX3)
is also promising, with bandgaps that occupy the range 1.5—2.2 eV.
Indeed, since the smallest bandgap approaches more closely the optimum value
for a single-junction cell than does methylammonium lead trihalide, it is
expected that higher efficiencies might be attained using these materials2.
It has
been shown that the bandgap in the methylammonium lead halides can be tuned by
varying the halide content, and2,3 that the diffusion lengths for
both holes and electrons in these materials are greater than 1 micron4.
The latter feature means that charges can be transported over long distances in
the perovskite, and the materials are effective in thin-film architectures. The
charges per se have been demonstrated
to comprise predominantly free electrons and holes, and are not bound excitons.
This is a consequence of the fact that the binding energy for excitons is sufficiently
low that charge separation can occur at room temperature5.

The
rise in power conversion efficiencies of solar cells based on perovksite
materials has been little short of meteoric, considering that just 3.8% was
achieved in 2009 6 but this had climbed to 22.1% in early 2016 7
(Figure 2). Working on the premise that even higher efficiencies should be
attainable and that the production costs of perovskite solar cells are relatively
low, they have attracted sufficient commercial interest that a number of start-up
companies have expressed their intention to be selling actual photovoltaic
modules by 2017 8,9. In view of the potential widespread importance
of perovskite solar cells in the future, it is most appropriate to include
mention of them in undergraduate courses. To this end, a very timely account
has been published which demonstrates how a simple such cell can be constructed
for the purposes of practical demonstration10.

Production of perovskite solar cells.

The processing of and fabrication of perovskite solar cells is
considerably less demanding than is the case for the more usual silicon-based
cells, for which costly multistep processes are necessary, needing to be carried
out in a clean room, involving high vacuum facilities, and temperatures in
excess of 1000 °C 11. In contrast, organic-inorganic
perovskites can be produced using wet chemistry techniques under standard laboratory
conditions. Thus, it has proved possible to synthesise the above mentioned
methylammonium and formamidinium lead trihalides by means of various solvent-based
and vapour deposition methods, with the indication that future production on
the large scale should be possible12,13. Despite the attractive
simplicity of the method, solution processing leaves voids, platelets, and
other defects in the perovskite layer, which might act to impede the photovoltaic
efficiency of the cell. To avoid such problems, a method has been presented14
that produces perovskite films with both a large crystal dimension and a smooth
structure, with more advantageous light conversion efficiencies. An alternative
approach, which involves solvent-solvent extraction at ambient temperature, enables
the generation of crystalline films which cover an area of several square
centimetres, in a controlled fashion, down to a thickness of 20 nanometers, and
without creating pin-holes.. An alternative method15 involves
annealing the spin coated, or exfoliated lead halide in the presence of
methylammonium iodide vapour at temperatures close to 150 °C. This offers
certain advantages over solution processing, since it may enable the creation
of multi-stacked thin films over larger areas16, as might be a
useful feature when it is desired to make multiple-junction cells. As a general
point, we note that vapour deposition methods yield more uniform thicknesses
than are created by simple solution processing. Both solution based and vapour
deposition approaches have their own advantages, and should prove scalable at
the manufacturing level, with much reduced costs and complexity in comparison
with making silicon photovoltaic cells. Solution methods are cheaper than
vapour based approaches, although the latter naturally eliminate some of the
need for solvents and having to deal with solvent residues. Planar thin film
layers can be created using both approaches, and either find application in
mesoscopic designs, for example to place coatings on a metal oxide scaffold, as
is typical for the perovskite or dye-sensitized solar cells that are currently
made. However, a major uncertainty over the use of perovskite solar cells
concerns their stability over the longer time, in view of the observation that
such materials undergo degradation under typical environmental conditions, with
concomitant losses in light conversion efficiencies.

Estimating the efficiency limit for perovskite
solar cells.

The Shockley–Queisser limit (detailed balance)17
for radiative efficiency is about 31% under an AM1.5G solar spectrum at
1000 W/m2, as applied to a Perovskite with a bandgap of 1.55 eV18.
This is somewhat less than the radiative limit of gallium arsenide (bandgap
1.42 eV) which can reach a radiative efficiency of 33%. A detailed balance
model for the maximal output power of a solar cell that can be achieved on the
proviso that certain conditions are met17:

(1) Carrier populations obey Maxwell–Boltzmann statistics. In
particular, the quasi-Fermi levels of electrons and holes must be split homogeneously
through the cell, where the magnitude of the “split” is equal to the applied
voltage. This assumes that the mobilities of the electrons and holes
(photocarriers) are sufficiently large, and indeed, charge carrier mobilities
up to 10 cm2 V−1 s−1 have been observed
in perovskites.

(2) The only existing recombination mechanism is that of a band-to-band
(monomolecular) process, while other nonradiative recombination processes, such
as Auger recombination and trap (defect)
assisted recombination can be ignored.

(3) The internal light conversion efficiency is 100%, such that for each
photon absorbed, one electron-hole pair is created; and in reverse, one photon
is produced for each recombination of an electron-hole pair. Indeed, an internal
quantum efficiency of close to 100% has been demonstrated for perovskites19.

(4) It is assumed that a photon recycling effect operates within the
cell. Although one photon is created by one electron-hole pair recombination, occurring
during the radiative recombination process, that photon can be reabsorbed
elsewhere in the cell, such that a new electron-hole pair is generated. This
photon reabsorption process can be enhanced so to achieve maximum solar cell
efficiency by using particular designs to achieve light trapping and angular
restriction. Nonetheless, whether such a photon recycling effect actually occurs
in perovskite solar cells has yet to be convincingly demonstrated.

The detailed balance model18 for the efficiency limit of CH3NH3PbI3
solar cells can be described by:

J(V) = Jt(V) + Je(V) - Jp

where VV {\displaystyle V} is the applied voltage
for the solar cell system, J p {\displaystyle J_{p}} Jp
is the photocurrent (photogenerated current) which results from the absorption
of incident sunlight, Je is
the current density loss from radiative emission, and JtJ e
{\displaystyle J_{e}} J t {\displaystyle J_{t}} Je and Jtjjis the loss
of current density as a result of trap-assisted nonradiative recombination.
Many approaches have been made, including analytical calculations, to identify
various relevant properties of perovskites, such as bandgap, effective mass,
and defect levels, while other structure-function methods have been explored as
a means to interpret the more detailed properties of these materials as they
might function in photovoltaic applications20.

Architectures for perovskite solar cells.

There are various different arrangements (“architectures”)
possible for the design of a perovskite solar cell (Figure 3), according to the
particular function of the perovskite material in the device, or the nature of
the top and bottom electrode. Where positive charges are to be harvested by a
transparent bottom electrode (cathode), such devices can broadly be classified
as being 'sensitised' (where the perovskite functions primarily as a light
absorber, and the subsequent charge transport takes place in other constituent materials),
or 'thin-film', where the majority of the electron or hole transport is
confined to the body of the perovskite per
se. In the sensitised version, the perovskite material is coated onto a
charge-conducting mesoporous scaffold as a light absorber, which
is typically TiO2,
and thephotogenerated electrons are transferred
from the perovskite layer to the mesoporous sensitised layer through which they
diffuse to the electrode and are delivered into the circuit. In the case of the
thin-film architecture21, both negative and positive charge carriers,
generated by light absorption, are transported through the perovskite layer to
charge selective contacts. Since the concept of dye-sensitized solar cells was
at the root of perovskite cells, initially it was the sensitized architecture
that was used, until it became recognised that better results could sometimes be
achieved by placing the materials in a thin-film architecture. In terms of longer
term stability, it is to be noted that sensitised architectures are likely to
suffer from degradation as induced by absorption of ultraviolet light, which
may limit the photovoltaic efficiency of such devices over time.

Stability of perovskite solar cells.

The stability of perovskite solar cells, under environmental operating
conditions, is critical. In this regard, the solubility of the organic
constituent of the absorber material in water is likely to cause a rapid
degradation in humid environments22. One potential solution to this
is to encapsulate the perovskite absorber as a composite along with carbon
nanotubes within a matrix of a relatively unreactive polymer: this, at least,
avoids an immediate deterioration of the material in moist air at elevated
temperatures22,23. Nonetheless, evidence is lacking that such
encapsulation methods would be successful over the longer timescale. A further
consideration, which applies to devices which contain a mesoporous TiO2
layer sensitised with the perovskite absorber, is a demonstrated instability to
UV-light24, which appears to be related to holes that are photogenerated
within the TiO2, along with oxygen radicals
that are formed on the surface of the material24. Thermal stresses
are another factor, and it is of note that the very low thermal conductivity determined
for CH3NH3PbI3 [0.5 W/(Km) at room temperature]
may limit the rapid transfer of heat through the material, and so provide some
protection against thermally induced stress and degradation over the working
lifetime of the solar cell25. Perovskite films contain a residue of PbI2
which has been shown to affect adversely the long term stability of
devices based upon perovskites14. In contrast, if a metal oxide
layer is used instead of the organic transport layer, it has been shown that
the cell can retain 90% of its photovoltaic capacity after 60 days26.
Clearly, however, this refers only to a quite short timescale and issues of
longer term stability remain.

Recent
Research on Lead Halide Perovskites for Solar Energy Conversion.

A representative overview of current
research in the field of perovskites for solar energy conversion devices is
given by a recently published special issue of Accounts of Chemical Research. The
following is a summary of some of the papers contained there. The first paper27
in the issue discusses organic–inorganic semiconductors with perovskite crystal
structures, which can be converted to efficient solar cells using
solution-processed active layers. Since these are made from earth-abundant
materials, and the necessary processing is not complicated, the likelihood is
that it will prove possible to manufacture them cheaply and on a large scale.
One critical difference between these materials and more traditional inorganic
semiconductors is that they furnish soft solids, in which a high degree of
thermally activated motion is present. The paper considers the internal motion
of methylammonium lead iodide (CH3NH3PbI3) and
formamidinium lead iodide ([CH(NH2)2]PbI3),
with particular emphasis on: (i) rotational-librational motion within the
cuboctahedral cavity; (ii) the spatial movement of large electron and hole
polarons; (iii) the transport of ionic defects bearing charges. As a
consequence of the above mentioned processes, various unconventional properties
are produced, which include frequency-dependent permittivity, low electron–hole
recombination rates, and current–voltage hysteresis27. Another paper28
explores the relationship between structural and electronic properties on a
molecular to mesoscopic scale for hybrid perovskites, some of which have been
demonstrated to have solar cell power conversion efficiencies exceeding 20%. It
is shown that, in hybrid lead-halide perovskites, dielectric screening is a
function of the local microstructure of the hybrid crystals, in addition to
basic chemical composition. It is concluded that band-gap engineering is
possible and the elementary photoexcitation dynamics can be controlled in order
to fine-tune the performance of the perovskites as they are present in
different optoelectronic devices28.

A theoretical approach29 is
employed to investigate the structural stability and electronic states of the
representative (110), (001), (100), and (101) surfaces of tetragonal CH3NH3PbI3,
taken to be a representative model for hole-transporting-material/CH3NH3PbI3
interfaces. Calculations, made on several different PbIx
polyhedron terminations, revealed the presence of two principal phases on all
of the four surface facets, which can by classified as vacant- and flat-type
terminations. Under conditions of thermodynamic equilibrium, the former are the
more stable. Evidence was obtained for a coexistence of both terminations on the
(110) and (001) surfaces. It was concluded that the surfaces may be partially
responsible for the relatively long carrier lifetime which is observed
experimentally for the perovskite solar cells, in which there are no mid-gap
surface states. It is reasoned that the shallow surface states on the (110) and
(001) flat terminations may furnish effective intermediates for hole transport
to hole-transporting materials, which may help to improve the overall performance
of perovskite solar cells29. Another paper considers rational
strategies for the fabrication of perovksite solar cells with high efficiency,
high stability and at low cost30. Important factors for the actual
achievement of these aims include the architecture, processing route and the
nature of the necessary materials employed in the construction of the solar
cells, which are usually constructed in the form of multilayer heterostructures
of light harvesting materials, of which electron and hole transporting layers
are critical components30. The structure and dynamics of hybrid
perovskite materials are considered31 in regard to the impressive
improvement in the power-conversion efficiencies in solar cells based on these
materials, which have risen above 20% in a mere 6 years. For a
solution-processable material, this is outstanding, and it is accordingly
essential to comprehend the physical and chemical phenomena that underpin the
photovoltaic efficiency processes. In the majority of attempts to understand
semiconducting materials and their optoelectronic and charge transport
properties, a model is often adopted in terms of a lattice of ions which are
displaced from their static positions by harmonic vibrations alone. However,
many recent studies indicate that this is an oversimplification of hybrid organic-inorganic
perovskite materials, where even at ambient temperature there is a range of dynamic
effects operating, in addition to those of fairly minor harmonic vibrations. In
their evaluation, the authors focus upon the mechanical “softness” of these particular
materials in comparison with other efficient photovoltaic materials, which
permits large ionic displacements to occur, even at ambient temperature. Accordingly,
their analysis extends beyond the usual simple static band structure model, in
regard to phase-transition behaviour and molecular/octahedral rearrangements. Atomic
diffusion phenomena are considered, particularly in terms of the migration of
intrinsic and extrinsic ionic species. Thus, hybrid organic-inorganic
perovskite are revealed as being highly dynamic in nature, whose charge-transport
and optical properties are influenced appreciably by structural fluctuations
and long-range motion of ions31.

Aspects are considered32 of
the so termed “inverted” planar device structure (i.e., p-i-n), for
inorganic–organic hybrid perovskite solar cells, in which p-type and n-type
materials are employed, respectively, as bottom and top charge transport
layers. Thus, power conversion efficiencies of up to18% have been demonstrated
for devices of this kind, which can be processed at lower temperatures, more
flexibility, and with only minor J–V hysteresis effects. A
comprehensive and critical overview is made of the according mesoporous and
planar structures, in addition to both the regular and inverted planar
structures. Further consideration is given to how the inverted planar structure
of perovskite solar cells has been developed, with particular attention being
made to such effects as film growth, band alignment, stability, and hysteresis32.
It has been emphasised33 that in order to optimise further hybrid
perovskite solar cells, a clearer understanding is necessary of how light is converted
into currents of electricity and also of how the reverse process functions. A
description is given of how optical-pump–terahertz-probe (OPTP)
photoconductivity spectroscopy may be used to investigate these phenomena since
it permits a monitoring of the dynamics of mobile charge carriers inside the perovskite
structure, which has been excited by means of a short laser pulse to deliver a
photon whose energy is within the solar spectral range. It is concluded that the
trapping of charges is the principal cause of mono-molecular
charge-recombination, where the trap depths are quite shallow (tens of
millielectronvolts). In contrast, bimolecular recombination is the result of
direct band-to-band electron–hole recombination. The latter significantly
violates the simple Langevin model. Values for the mobility of charge-carriers,
as obtained from OPTP measurements and how these depend on both the composition
and morphology of the hybrid perovskites are discussed33.

In another
paper34, published experimental evidence on the photophysics of
perovskites is examined within a coherent theoretical framework, based upon
well established knowledge of materials optoelectronics. One critical unknown
is whether photon absorption leads to a population of unbound, conductive free
charges or rather excitons, formed as a result of a Coulomb interaction
occurring immediately below the band gap energy. It is concluded that all
hybrid halide perovskites behave as free-charge semiconductors, and it
is this feature, along with a range of band gap energies that encompass the
whole solar spectrum, which determines perovskites as materials from which highly
efficient, single and multijunction solar cells can be made34. Also
included in the series is a paper which explores the physical changes that may
occur in perovskites that have undergone illumination over a prolonged period35.Three model systems were employed: (i) a
free-standing perovskite film on a glass substrate, (ii) a symmetrical system
with nonselective electrical contacts, and (iii) a working perovskite solar
cell (either a planar or a porous structure).The authors make a critical
distinction between photoinduced effects and photo- and electric-field induced
effects, and it is demonstrated that the photoinduced effects are large and of
great significance to the properties of the perovskite materials35.

In another paper36 is
considered the lifecycle of organic–inorganic lead halide perovskites,
especially in regard to (i) the nature of the precursor solution, (ii) the
formation of solid-state perovskite thin films and single crystals, and (iii)
the transformation of perovskites into hydrated phases upon exposure to
moisture. It is shown that information about the thermally driven creation of
the perovskite structure is readily provided by a range of spectroscopic and
structural characterization techniques. It is concluded that in order to make stable,
high-quality perovskite thin films for future generation photovoltaic and light
emitting devices, a fundamental understanding of lead halide perovskite
formation and degradation pathways is advantageous36. Another paper
considers the effects of light and electron-beam irradiation on hybrid lead
halide perovskites and their solar cells37. Electron beams are used
in the characterization of these devices but radiation damage is incurred,
meaning that such exposure should be kept to a minimum. While some overall
picture of the effects of light on these materials and devices is possible, it
is not as yet feasible to predict the longer term stability of these solar
cells, under actual working conditions37.

In its July 2016 issue, the flagship
magazine of the Royal Society of Chemistry, “Chemistry World”, published an
overview38 of the current state of the art for solar cell
technologies, including perovskites. It is proposed that the latter can be made
in thin-film forms and are sufficiently flexible to manufacture using a
roll-to-roll process. It is also possible to boost the performance of
conventional silicon solar cells by layering perovskites onto them. There is
research being undertaken to reduce the potential environmental toxicity of
perovskites by eliminating lead as one of the elemental components. A potential
solution to the problem of degradation of the cell from hydrophobic lithium
salts, which are used as a hole-transport dopant, is to use instead a
hydrophobic hole-transporter in conjunction with a doping technique that
employs a pre-oxidised salt of the respective hole-transporters, to produce
perovskite cells that are efficient in their light-to-power conversion but much
less moisture sensitive. A computer screening has been made of over 4 trillion
possible compounds in an effort to identify the next generation of inorganic
photovoltaic materials, the data from which have yet to be fully analysed.

Figure
2. Solar cell efficiencies of various
cell technologies as tracked by the U.S. National Renewable Energy Laboratory
(NREL). http://www.nrel.gov/ncpv/images/efficiency_chart.jpg

Credit:
U.S. Department of Energy.

Figure 3. Schematic of a sensitized
perovskite solar cell in which the active layer consist of a layer of mesoporousTiO2
which is coated with the perovskite absorber. The active layer is contacted
with an n-type material for electron extraction and a p-type material for hole
extraction. b) Schematic of a thin-film
perovskite solar cell. In this architecture in which just a flat layer of
perovskite is sandwiched between to selective contacts. c) Charge generation
and extraction in the sensitized architecture. After light absorption in the
perovskite absorber the photogenerated electron is injected into the mesoporous
TiO2 through which it is extracted. The concomitantly generated hole
is transferred to the p-type material. d) Charge generation and extraction in
the thin-film architecture. After light absorption both charge generation as
well as charge extraction occurs in the perovskite layer.

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